Ban relationship which is formal (5,3)
I believe the answer is:
black tie
'is formal' is the definition.
(I've seen this before)
'ban relationship' is the wordplay.
'ban' becomes 'black' (I've seen this in other clues).
'relationship' becomes 'tie' (tie is a kind of relationship).
'black'+'tie'='BLACK-TIE'
'which' is the link.
(Other definitions for black tie that I've seen before include "One accompanies DJ" , "Accompaniment of 11" , "Formal evening wear for men" , "Formal (neck)wear" , "It's worn with a dinner jacket" .)